Dicrotonic esters



v Patented May 23, 1950 DICROTONIC ESTERS Don E. Floyd, Minneapolis, Minn., assignor to General Mills, Inc., a corporation of Delaware No Drawing. Application July 10, 1948, Serial No. 38,162

7 Claims.

The present invention relates to improvements in the process of converting crotonic esters to esters of alpha-ethylidene-beta-methyl glutaric acid, commonly called dicrotonic esters. This application is a continuation-in-part of my copending application, Serial No. 632,597, filed December 3, 1945, now abandoned. Dicrotonic acid and its esters are useful as plasticizers and as intermediates for the preparation of condensation polymers, alkyd resins, interpolymers with other unsaturated compounds, and as intermediates in general organic syntheses. The preparation of dicrotonic acid and its esters by condensation of crotonic esters has been known. For example, Von Pechmann, Ber. 33, 3323 (1900) prepared the dimethyl ester of dicrotonic acid by warming 3, mixture of ether, methyl crotonate, and an equi-molecular amount of sodium methylate for a period of three to four hours. He claimed a yield of 60-70% of dicrotonic acid dimethyl ester. No data is given as to the purity of the product obtained. Actual repetition of Von Pechmanns work indicates a yield of around 43% based on methyl crotonate fed to the reaction, and about 55% based on methyl crotonate consumed. This method entails definite disadvantages in that it involves the use of equi-molecular amounts of sodium methylate, a catalyst which is expensive to prepare and use. Furthermore, the yields are low, particularly as a result of certain side reactions such as acyloin condensation, acetoacetic ester type self-condensation, reduction by sodium, polymerization, and addition of the alcoholate to the double bond in the crotonic ester. The by-products resulting from these side reactions not only reduce the yield, but actually contaminate the product obtained, thereby necessitating extensive purification.

Another attempt at the production of dicrotonic acid and its esters is reported by Michael, Ber. 33, 3731 (1900), who allowed a mixture of ether, ethyl crotonate, and approximately half the molar amount of metallic sodium to stand at room temperature for three days. He reported a yield of about of dicrotonic acid diethyl ester. This procedure entails essentially the same disadvantages as those-reported by Von Pechmann. Moreover, the extended time period required is disadvantageous from a production standpoint, and at the same time permits side reactions to proceed to a greater extent.

It has now been discovered that it is possible to carry out this condensation under conditions which permit it to proceed to substantial completion in a relatively short period of time, while at u then added and 2 the same time utilizing only a relatively small amount of catalyst, effecting high yields of a product, requiring a minimum of purification, and yielding a minimum of side reaction products.

It is, therefore, an object of the present invention to provide a process of preparing dicrotonic acid and its esters in high yield with a minimum amount of by-products in a comparatively short period of time without the necessity of extensive purification and with the consumption of relatively small quantities of catalyst.

Other objects will be more fully apparent from the following description of the invention.

The invention consists of carrying out the condensation of crotonic esters at an elevated temperature preferably in the presence of a high boiling solvent and in the presence of a small amount of catalyst, for a comparatively short period of time. Temperatures preferably in excess of C. and up to 200 C. or higher are suitable. in the absence of a solvent but are preferably employed with a solvent in view of the difliculty of maintaining temperature control in the absence thereof. High boiling, inert solvents such as aromatic and aliphatic hydrocarbons, ethers, and the like may be employed. By operating at elevated temperatures and by using small quan-. tities of catalysts, side reactions have been reduced to a minimum or eliminated completely and a, more complete condensation to the dicrotonic ester obtained. The solvents employed can be recovered practically quantitatively by distillation. They are not as subject to loss through volatilization or to fire hazard as is the case with ethyl ether. Moreover, they are readily obtained commercially in an anhydrous condition and free of interferring substances such as alcohols. Not only has the yield in this condensation been improved, but the purity of the reaction product has also been improved. Conversion of the crotonic ester is nearly complete while at the same time competing side reactions are minimized. This eliminates unchanged starting material and certain side reaction products as impurities. Therefore, extensive purification by fractionation or by other means is unnecessary.

The reaction is carried out by mixing the crotonate with a solvent in a flask fitted with a mechanical stirrer and condenser protected by a drying tube. A small amount of catalyst is the mixture stirred and refluxed Such temperature may be employedfor a period of several hours. By this time the reaction is substantially complete as evidenced by the absence of the pronounced odor oi the crotonate. The reaction mixture is then cooled and shaken with dilute acid to efiect removal of the alkaline catalyst. The removal of the last traces of acid, alkali, or salts may be effected by water-washing and then drying. Following distillation oi thesolvent, the dicro'tonate product may be vacuum distilled. In order to obtain a product of utmost purity the dicrotonate may be subjected to a fractional distillation. As an alternative, the ester may be hydrolyzed'to the free dicrotonic acid which may then be obtained in the pure form by crystallization.

A period of three to twelve hoursis usually required for the reaction. With very'active catalysts, such as powdered sodium hydride, usually three to four hours is sufficient, while with less active catalysts more time is required. The reaction temperature also has a pronounced effect on the-reaction andfinlgeneral-a high temperature promotes morerapid reaction. Finely powdered catalysts which are readily dispersed also promote more rapid reaction than those with coarser particle size.

Several variations arepermissible in the various factors. involved in the reaction. For example, the crotonate. employed may be an ester containing an aliphatic, aromatic, aryl-aliphatic, alicyclic, or heterocyclic alcohol residue which may be saturated or unsaturated and may or may not contain substituent groups which do not react with or interfere with the catalyst. In general, as the groups become larger, the reactivity of the ester is less and accordingly the reaction rate is lower. Accordingly, it is .preferred to employ lower aliphatic-esterssuch .asmethyl, ethyl, propyl, butyl, and the like.

The solvents employed should be relatively inert toward the reactants and have a medium to high boilingpoint. In general, aromatic hydrocarbons, aliphatic hydrocarbons, aryl-aliphatic hydrocarbons, .and ethers, both symmetrical and mixed, may be employed. Examples of such solvents, are benzene, toluene, xylene, dibutyl etheighigh boiling petroleum solvents, and the-like. Thetime of reaction will depend upon a number of factors suchas'the solvent used, temperature oil-reaction, andthe catalyst. Since it is ordinarily desired to :securea quantitative conversion of thecrotonic ester, aheatingshouldbe-continned until the pronounced odor of the crotonic esterhas disappeared. Another way of detecting substantial completion of the reaction. is, to. watch thetemperature, which *willgradually rise. untilthe conversion is completesat whichpoint the temperature will remain fairly constant, or the ratecf temperature @risewill be reduced considerably. Prolonged heating beyond the point of reaction will reducethe yieldo-f the :dicrotonic ester whileincreasing the amount of polymerization products.

Various catalysts are-satisfactory for the reac- I tion. They include metallic sodium and potassium, sodium and potassium methoxide, ethoxide, and the like. In general, the alkali metals and their basic derivatives such as alkoxides, amides, hydrides, alkyls, and the like, are most suitable. Certain other metals and their derivatives possess someactivity as is wellknown by those-skilled in the art. lllfhen metals suohas sodium are used as catalysts the principal competing reactions are acy-loin condensation, .acetoacetic type selfcondensation, and reduction by the metal. In

order to eliminate or minimize these reactions it is important to use very small amounts of metallic catalyst. When metal alcoholates are used as the catalyst the principal competing reactions are the production of beta-alkoxybutyric esters by addition to the double bond in the crotonic ester and the acetoacetic ester type self-condensation. In thiscase theme of a smal-l amount of catalyst eliminates or minimizes these side reactions.

As to the amount of catalyst, in general it has been found that decreasing the amount of cata- .lyst below 0.15 mole per mole of crotonic ester results in material increases in yields. Thus at -cataly'st mole -ratios ofi'about 0.15 per mole of crotonic -ester-yieldsofapproximately 67% are obtained. At a catalyst mole ratio of approximately '0.1',.a;yield of'about 74% is obtained. At catalyst mole ratios below 0.1 yields within the range of hours; By this time it -hau become dee brown in colorand the *pronouncedodorof ethyl crotona'te had disappeared. It wasfcooledand-shaken with dilute sulfuric acid and then washed with water and dried. The-benzenewas-distilled ofi andtheester product distilled inv'acuuin. I293 grams of product boiling 13 7 9 -C.'/i7 was obtained. A portionp'f tl'iises'ter was hydrolyzed to th'edicrotoriic acid, :wl'iicli'melted at I2S '9-C. Its neutra-ieouivalent-was scz Example 500' grams. of dry xylene -(comi'nercial xylol') and .500 g. of :ethylcrotonatewere :refluxed and stirred tvithtS: g. of afresh sodium :methoxide .for' six hours. Duringthisitime th'ereaction was protected-limit; the' moisture ref tl'ie' air bye. calcium chloride tube. The 'mixtinte was cooled and a stirred well with dilute aceticacid, then: washedand worked 11;) as'..1 n the zp-revious case. Theyield was"4431mmidicrotoniciester produet,boiiing (at 145-150" C'.-/.'20- mm;- Some-of ithe ester was thydrolyzed to give the some -:dicrotonic acid. as before.

Example. 3.

Fifty "grams -"of "methyl crotonate 'and '50 g. or a petroleum ether marketed commercial-1y underthe name, 'Sk'ellysolveC,*wel'erefiuxedwith 1 g.- off metallic sodiuzirfor eight hours with stirring and. with the usual precautions against moi'sture.

After the reaction product had' bee'n worked up. 1

ten grams of flnhafigld methyl 'crotonate was recovered and 34 g. er the di'croto'nic esters was produced. Boiling point -4 Jo 16 'mm'.

'Erample 4. Fifty grams-10f :ethylicrotonatesand 50 goof dry; toluene swereirefiuxed with :1 of sod-ium ethylate in the .manner described above, .f or' five hours.

The product was aeidified. and washed in the" usual manner; Forty -one and SBVGIIFlZfiDllhS grams of dicrotonic ester product boiling at 139-144 C./18 mm. was obtained.

Example Seventy-five grams of butyl crotonate and 75 g. of xylene were refluxed with 1 g. of metallic sodium for eight hours. The mixture was cooled and stirred with dilute sulfuric acid. After washing and drying, the solvent was distilled ofi and the ester product distilled in vacuum. Sixty-five and eight-tenths grams of distillate boiling at 146-154 C. at 18 mm. was obtained. There appeared to be no unchanged butyl crotonate.

Example 6 Fifty grams of dibutyl ether and 50 g. of methyl crotonate were refluxed for eight hours with metallic sodium as catalyst. The reaction product was treated in the usual manner as described previously. Thirty-six grams of the dicrotonic methyl ester, boiling at 120-4 C./16 mm. was produced.

Example 7 Fifty grams of ethyl crotonate and 50 g. of xylene were refluxed with 1 g. of metallic potassium freshly cut in small pieces. The reaction mixture was mechanically stirred and protected from moisture by a calcium chloride tube. After seven hours the mixture was cooled and worked up as in the previously described examples. There was obtained 41 g. of the dicrotonic ester product which was found to boil at 145-150 C./20 mm.

Example 8 One hundred grams of methyl crotonate and 100 g. of benzene were refluxed with 5.4 g. of powdered sodium methoxide for a period of eight hours. The reaction mixture was mechanically stirred and protected from moisture in the air by means of a condenser fitted with Drierite tube. Then the mixture was cooled to room temperature and shaken with 4% acetic acid and finally washed with water and dried over anhydrous sodium sulfate. The benzene was distilled off and then the ester product was separated by fractional distillation. There was recovered 6.1 g. of methyl crotonate and there was obtained 69.2 g. of methyl dicrotonate. A dark residue weighing 8.8 g. was left after distillation. The methyl dicrotonate had a boiling point of 128-134" C./24 mm. The 11 was 1.4508.

Example 9 One hundred grams of methyl crotonate and 100 g. of benzene were refluxed with 8.1 g. of powdered sodium methoxide for a period of eight hours. The reaction mixture was mechanically stirred and protected from moisture by means of a condenser fitted with a Drierite tube. Then the reaction mixture was cooled to room temperature and shaken with dilute acetic acid and finally washed with water and dried over sodium sulfate. The benzene was distilled off and then the ester product was separated by fractional distillation. There was recovered 3.1 g. of low-boiling distillate consisting mainly of methyl crotonate and there was obtained 65.8 g. of methyl dicrotonate. A dark residue weighing 9.8 g. was left after the distillation, The methyl dicrotonate boiled at 128-133 C./24 mm. The m was 1.4509.

I claim as my invention:

1. Process of condensing a crotonic ester to a dicrotonic ester, which comprises subjecting the crotonic ester in the liquid phase to a temperature within the approximate range of -200 C. in the presence of a condensation catalyst selected from the group consisting of alkali metals and alkali -metal alkoxides in a mole ratio below 0.15 based on the crotonic ester.

2. Process of condensing a crotonic ester to a dicrotonic ester, which comprises subjecting the crotonic ester dissolved in a high boiling solvent which is relatively inert to the reactants, in the liquid phase to a temperature within the approximate range of 70-200 C. in the presence of a condensation catalyst selected from the group consisting of alkali metals and alkali metal alkoxides in a mole ratio below 0.15 based on the crotonic ester.

3. Process of condensing a low aliphatic crotonic ester to a dicrotonic ester, which comprises subjecting the low aliphatic crotonic ester in the liquid phase to a temperature within the approximate range of YO-200 C. in the presence of a condensation catalyst selected from the group consisting of alkali metals and alkali metal alkoxides in a mole ratio below 0.15 based on the low aliphatic crotonic ester.

4. Process of condensing a crotonic ester to a dicrotonic ester, which comprises subjecting the crotonic ester in the liquid phase to a temperature within the approximate range of 70200 C. in the presence of a condensation catalyst selected from the group consisting of alkali metals and alkali metal alkoxides in a mole ratio not substantially in excess of 0.1 based on the crotonic ester.

5. Process of condensing a crotonic ester to a dicrotonic ester, which comprises subjecting the crotonic ester in the liquid phase to a temperature within the approximate range of 70-200" C. in the presence of an alkali metal condensation catalyst in a mole ratio below 0.15 based on the crotonic ester.

6. Process of condensing a crotonic ester to a dicrotonic ester, which comprises subjecting the crotonic ester in the liquid phase to a temperature within the approximate range of 70-200 C. in the presence of an alkali metal alkoxide condensation catalyst in a mole ratio below 0.15 based on the crotonic ester.

7. Process of condensing a low aliphatic crotonic ester to a dicrotonic ester, whichcomprises subjecting the low aliphatic crotonic ester in the liquid phase, while dissolved in a high boiling aromatic hydrocarbon solvent which is inert to the reactants, to a temperature within the approximate range of 70-200 C. in the presence of not substantially in excess of 0.1 mole of a condensation catalyst selected from the group consisting of alkali metals and alkali metal alkoxides, based on the low aliphatic crotonic ester.

DON E. FLOYD.

REFERENCES CITED The following references are of record in the file of this patent:

Von Pechmann, Berichte 33, 3323-3341 (1900). Michael, Berichte 33, 3731-3769 (1900).

Certificate of Correction Patent No. 2,508,910 May 23, 1950 DON E. FLOYD It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 4, line 68, for 130-4 (1/16 mm. read 1204 0./16' mm;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office. Signed and sealed this 29th day of August, A. D. 1950.

THOMAS F. MURPHY,

Assistant C'ommz'saz'oner of Patents. 

1. PROCESS OF CONDENSING A CROTONIC ESTER TO A DICROTONIC ESTER, WHICH COMPRISES SUBJECTING THE CROTONIC ESTER IN THE LIQUID PHASE TO A TEMPERATURE WITHIN THE APPROXIMATE RANGE OF 70-200*C. IN THE PRESENCE OF A CONDENSATION CATALYST SELECTED FROM THE GROUP CONSISTING OF ALKALI METALS AND ALKALI METAL ALKOXIDES IN A MOLE RATIO BELOW 0.15 BASED ON THE CROTONIC ESTER. 